A mechanical joint assembly is an assembly which allows two members to be mechanically joined but allows rotation of one member with respect to the other about one or more rotation axes. A universal joint is a mechanical joint assembly providing rotation about two substantially orthogonal axes. Such a joint is useful in applications in which two members must be mechanically joined but must be allowed to assume an arbitrary relative angle.
In some applications it is desirable for the motion of the moving members about the universal joint to be restricted or damped by braking. For example, such a braked universal joint is useful in situations wherein a first joined member is a boom with a second joined member suspended therefrom being some load carrying means, wherein motion of the load carrying means must be restricted, particularly when not loaded. One particular application of a braked universal joint is suspension of a grappler from a boom of a logging skidder. Several previous designs for a braked universal joint used in this way (also referred to as a swivel link) are described in U.S. Pat. Nos. 4,335,914; 4,417,759; 4,573,728; 4,679,839; 4,715,641; 5,110,169; 5,451,087; and 5,601,161, each of said patents being incorporated by reference as if fully set forth herein. A much improved design for a swivel link incorporating frusto-conical surfaces for braking and load-bearing is disclosed in U.S. Pat. No. 5,713,688 issued to the applicant of the present application, and co-pending application Ser. No. 08/909,552 now (U.S. Pat. No. 5,779,383) filed in the name of the applicant of the present application, both of said patent and said application being incorporated by reference as if fully set forth herein. The design and construction of swivel link assemblies, many drawbacks of previous swivel link designs, and improvements resulting from the use of frusto-conical braking and load-bearing surfaces are fully disclosed therein, and need not be reiterated herein.
A primary figure-of-merit for a swivel link is the number of hours of use in the field before replacement of the friction members of the joint (friction discs in older designs, friction cones in the frusto-conical design). Anything that reduces wear of the friction members (and therefore reduces concomitant down time, maintenance time, and maintenance costs) is highly desirable.
During use of a swivel link, and many other braked mechanical joints, it is often the case that small motions about the joint are insignificant and can be tolerated, while larger motions are undesirable and must be prevented (for example, to prevent injury or equipment damage). However, previous joints provide a constant braking torque independent of the amount of motion which has occurred. Reduction of the braking torque for small motions and/or motions near a rest position would reduce unnecessary wear on the friction members, "saving" the friction members for suppression of larger amplitude motions and/or motions far from the rest position.
It is therefore desirable to provide a braked mechanical joint assembly in which a relatively smaller braking torque is applied during small amplitude motions and/or motions near a rest position of the joint, while a relatively larger braking torque is applied during large amplitude motions and/or motions far from the rest position of the joint. It may also be desirable to provide a braked mechanical joint assembly in which the braking torque may vary with joint position and/or motion in an arbitrary fashion.